Unlike diagnosis in adults, definitive diagnosis of HIV in children less than 18 months of age requires virologic testing. Maternal IgG antibodies cross the placenta into the fetal circulation through pregnancy and are also secreted in breast milk. As infants may retain these maternal antibodies for up to 18 months after birth, conventional HIV antibody tests are of limited use in determining positive infection status. Instead it is necessary to directly identify HIV infection using a virologic method, typically polymerase chain reaction (PCR) testing to detect viral DNA or RNA.